Thanks for your postings. The Priority values and other fields can be changed in the administration area under Administration > Customizations.
On the same page, you can also add and configure the custom fields for TestRail. You can add custom fields for test cases (for the Test Suites & Cases tab) and tests/test results (which appear on the Add Test Result Dialog). You can create custom fields with several different types such as a Dropdown, Text, Date or simple String/Number fields (and soon a multi-select custom field). There’s also a custom field which adds support for a very nice Steps control (useful if you prefer a more structured approach to entering test case steps):
I hope this helps and please let me know in case you have any further questions.